Contact printing, such as Gravure printing, is an industrial printing process mainly used for the high speed production of large print runs at constant speed and high quality. It is understood that the gravure process is utilized to print millions of magazines each week, as well as mail order catalogues and other printed products that require constant print quality that must look attractive and also demonstrate exactly what they offer. Examples of contact printed products include art books, greeting cards, advertising, currency, stamps, wallpaper, wrapping paper, magazines, wood laminates, and some packaging.
Gravure printing, a de-facto sub-set of contact printing, is a direct printing process that uses a type of image carrier called intaglio. Intaglio means the printing plate, in cylinder form, is recessed and consists of cell wells that are etched or engraved to differing depths and/or sizes. These cylinders are usually made of steel and plated with copper and a light sensitive coating. After being treated, the gravure cylinder is usually machined to remove imperfections in the copper.
Most gravure cylinders are now laser engraved. In the past, gravure rolls were either engraved using a diamond stylus or chemically etched using ferric chloride. If the cylinder was chemically etched, a resist (in the form of a negative image) was transferred to the cylinder before etching. The resist protects the non-image areas of the cylinder from the etchant. After etching, the resist is stripped off. Typically, following the engraving process, the cylinder is proofed and tested, reworked if necessary, and then chrome plated. Today, corrections to laser engraved gravure cylinders are performed using the old chemical etching process.
As shown in FIG. 1, contact printing systems using direct image carriers, such as gravure cylinders, apply an ink directly to the gravure cylinder (also known as a central roll). From the gravure cylinder, the ink is transferred to the substrate. Modem gravure presses have at least two gravure cylinders 100, 100A that rotate in a respective ink bath 118, 118A where each cell of the design imposed upon the surface of the gravure cylinders 100, 100A is flooded with ink. A system called a doctor blade 106, 106A is angled against the gravure cylinder 100, 100A to wipe away the excess ink leaving ink only in the cell wells of each respective gravure cylinder 100, 100A. The doctor blade 106, 106A is normally positioned as close as possible to the nip point of the substrate 100 meeting the respective gravure cylinder 100, 100A. This is done so ink in the cells of the gravure cylinder 100, 100A has less time to dry out before it meets the substrate via the respective impression rollers 102, 102A. The capillary action of the substrate 110 and the pressure from the impression rollers 102, 102A draw and/or force the ink out of the cell cavity of the gravure roll 100, 100A and transfer it to the substrate 110.
What is important to understand is that typical gravure systems provide for a plurality of individual gravure stations where each gravure cylinder supplies an individual ink to the web substrate 110. Thus, in order to provide a finally printed product 112, 114, 116 having eight colors, a gravure printing system will require eight individual gravure stations. Similarly, a finally printed product 112, 114, 116 having five colors would require a gravure printing system having five individual gravure stations. Sequentially, a web substrate 110 will pass between a first gravure cylinder and a first impression cylinder 102 which transfers a first ink to the web substrate 110 which is then dried in a dryer 104 prior to application of a second ink from the combination of a second gravure cylinder 100A and second impression cylinder 102A. The subsequent printed product is then dried in a second dryer 104A and subsequently converted into a final product in the form of a convolutely wound roll 116, a folded product 114, or a stack of individual products 112.
It should also be noted that it is required that the ink applied to the web substrate 110 is dried before the web substrate 110 reaches the next printing station of the gravure system. This is necessary because wet inks cannot be overprinted without smearing and smudging. This emphasizes the need for high volume drying equipment such as dryers 104, 104A to be placed after each gravure printing station.
The printing impression provided to web substrate 110 and produced by the gravure processes are accomplished by the transfer of ink from cells of various sizes and depths that are etched onto the gravure cylinder 100, 100A as shown in FIGS. 2A-2C. The respective cells 120A, 120B, 120C can be provided in different sizes and depths, and the gravure cylinder 100, 100A may contain as many as 22,500 cells per square inch. The various sizes and depths of the depressions of the cells 120A, 120B, 120C create the different densities of the image. A larger or deeper depression transfers more ink to the printing surface on web substrate 110, thereby creating a larger and/or darker area. The regions upon gravure cylinders 100, 100A that are not etched become non-image areas. Further, the cells 120A-120C that are engraved into the gravure cylinders 100, 100A can be different in area and depth, or they can be the same depth but different in area. This can allow for greater flexibility in producing high quality work for different types of applications. Cells 120A-120C that vary in area but are of equal depth are often used on gravure cylinders 100, 100A for printing packaging applications. Gravure cylinders 100, 100A with cells 120A-120C that vary in area and depth are typically reserved for high quality printing. It is understood that printed images produced with gravure are high quality because the thousands of ink cells 120A-120C appear to merge into a continuous tone image.
Besides being very thin and fluid, the ink colors used with the gravure process color applications typically differ in hue than the inks used with other printing processes. Instead of the usual cyan, magenta, yellow, and black hues used with offset lithography, blue, red, yellow, and black are typically used. Standards have been established by the Gravure Association of America for the correct types of inks and colors that should be used for the different types of substrates and printing applications.
However, as can be seen, the gravure process can be costly and requires numerous gravure printing stations in order to provide a web substrate with several colors and images that require a large gamut. As mentioned previously, providing an image onto a web substrate that has eight colors typically requires eight gravure print stations. The gravure apparatus is costly to produce due to the nature of producing the individual gravure rolls. Additionally, the ancillary equipment required by the gravure process (e.g., doctor blades, impression cylinders, and dryers) adds to the cost of a single gravure station. Multiply this cost over the need to produce high definition, high quality, and multi-color images running a large color gamut increases the associated equipment costs accordingly. Further, the floor space footprint of a single gravure station is typically quite significant. If this is multiplied by the several stations required to print several colors onto a web substrate, the amount of floor space required is accordingly increased.
Thus, it would be advantageous to not only provide a contact printing system such as a gravure printing system that can provide the application of several different inks onto a single web substrate with a single gravure roll but also reduce the floor space required for such a printing system.